The efficacy of radiation therapy in the treatment of brain tumors is limited by the toxicity of ionizing radiation to the surrounding normal tissue. In the rat model of cerebral radiation injury, pentobarbital has been shown to enhance overall survival in a dose-dependent manner. Evaluation of alternative barbiturates reveals that thiopental is of equivalent radioprotective value to pentobarbital. The rodent model of radiation injury does not parallel that of human injury. A primate model was designed to assess the role of pentobarbital in circumstances more applicable to the human situation. This ongoing study has thus far revealed the ability of pentobarbital to limit the toxicity of the radiation utilized. Neuroendocrinologic evaluation has revealed early dysfunction of thyroid-stimulating hormone, luteinizing hormone, and prolactin responses to stimulatory testing in the animals irradiated while anesthetized with ketamine. Significantly less abnormalities have occurred in the pentobarbital group.